This invention relates generally to transmitting optical signals, and more particularly, to reducing pulse broadening in optical fibers and to optical clocks.
FIG. 1 shows that an initial optical pulse 2 becomes a broader pulse 3 after traveling through an optical fiber 4. The broadening of the pulse 2 results from dispersion. One reason for dispersion is the variation of a fiber""s index of refraction with wavelength. The index of refraction variations make longer and shorter wavelength components of the pulse 2 travel at different speeds in the optical fiber 4. After traveling through a certain length of the optical fiber 4, the speed variations produce the broader pulse 3. Another reason for dispersion is waveguide dispersion, which is induced by the geometric configuration of the fiber 4.
Pulse broadening can affect the quality of digital data transmission in the optical fiber 4. Digital data is transmitted as a series of optical pulses. Each temporal interval for a source pulse represents one binary bit. The binary states xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d correspond to the presence and absence of a pulse, respectively. As pulses broaden and overlap, a receiver may not be able to determine whether a pulse is present in a particular time interval or whether a detected optical signal is the tail of a previous or subsequent pulse. Inserting an amplifier 5 into the optical fiber 4 can aid to reduce receiver errors due to propagation weakening of pulse intensities. But, the amplifier 5 does not aid to reduce receiver errors caused by the dispersion generated pulse broadening and overlap.
Present optical fiber communications use optical pulses having wavelengths of about 1.5 microns, because erbium-doped fibers can provide quality optical amplification at 1.5 microns. Unfortunately, many older optical fibers produce significant chromatic dispersion in optical signals at 1.5 microns. This chromatic dispersion produces significant pulse broadening, which limits transmission wavelengths and distances in contemporary optical networks.
In one aspect, the invention provides a method for transmitting optical pulses in a transmission medium. The method includes separating a coherent source optical pulse into a plurality of mutually coherent pulses, and producing a series from the plurality of pulses. The series is transmitted through the medium, and the pulses of the series are received at a distant region of the medium. The series of pulses is adapted to interfere and form a packet whose width is narrower than the width of any pulse of the series at the distant region of the medium.
In preferred embodiments, the method further includes dividing the source pulse into a plurality of pulses directed into different directions, sending the pulse directed in each direction through a separate optical waveguide to produce a delayed output pulse, and recombining the output pulses to produce the series. Each output pulse has a different delay.
In a second aspect, the invention provides an apparatus for transmitting an optical signal in an optical fiber. The apparatus includes an optical beam splitter to split a source light signal into a plurality of separated, mutually coherent light signals, an optical train to produce a series of mutually coherent, outgoing light signals from the separated signals, and a combiner. Each outgoing light signal has a different time delay. The combiner receives the outgoing light signals from each of said conduits and is adapted to redirect the received light signals into an optical fiber.
Embodiments of the invention provide methods and apparatus for quasi-dispersionless optical communications links. Some embodiments compensate dispersion induced pulse broadening in optical fibers. The reduced pulse broadening enables higher data rate transmission in long optical fibers. Other embodiments compensate dispersion induced pulse broadening occuring in free space propagation of optical signals.